Physical properties of young brown dwarfs: High-resolution near-infrared spectroscopy of young M and L dwarfs

Brown dwarfs are objects with masses intermediate between stars and planets (0.072 M⊙ > M > 0.012 M⊙ ). As the lowest-mass objects that form like stars, brown dwarfs are important for understanding the low-mass end of the initial mass function, the complete mass dependence of processes in star formation, and the relationship between stellar and planetary atmospheres. Brown dwarfs are relatively recent additions to the menagerie of stellar astrophysics, and their cool atmospheres cause complex spectral morphology. As such, their observational characteristics are not yet unambiguously related to fundamental physical properties, which are extremely difficult to measure directly.;Using the W. M. Keck Observatory, I observed a large and unique sample of old and young brown dwarfs at high spectral resolution in the near-infrared. The sample ranges from young, hot M dwarfs to cool, old L dwarfs, corresponding to a range of effective temperatures and surface gravities. The near-infrared J band (∼1.1--1.4 mum) corresponds to the wavelength regime at which all but the coolest brown dwarfs emit the largest fraction of their flux, and J-band spectra of brown dwarfs contain a variety of atomic and molecular features that are sensitive to temperature and gravity. The wavelength coverage and spectral resolution of the NIRSPEC near-infrared cross-dispersed echelle spectrometer combined with the collecting area of the Keck II 10-meter telescope result in an unparalleled observational dataset.;To complement the observations, I model brown dwarf atmospheres using the PHOENIX code with updated atomic abundances and molecular line lists. I calculate converged structures with physical parameters appropriate for the observed sample at a finer sampling in effective temperature and surface gravity and create synthetic spectra at higher resolution than previous studies. This allows a detailed examination of the correspondence between observed and synthetic spectra as well as the evaluation of successes and failures of the models.;I determined best-fit model parameters (effective temperature, surface gravity, radial velocity, and projected rotational velocity) for a subset of observed spectra spanning a range of spectral types and ages. This is the first and most complete comparison of state-of-the-art synthetic near-infrared spectra with high-quality spectral observations to derive physical properties and identify deficiencies in the models. Results from this approach are shown to be consistent with other derivations of physical parameters, and they establish a road map for optimizing spectral observations of brown dwarfs and for improving models of cool atmospheres.